The present invention relates to an address mark detection system for a magnetic disk drive and for finding the head of a read data signal and, more particularly, to an address mark detection system which uses data erase portions as address marks and ensure accurate detection against noise introduced in the erase portions.
In a magnetic disk drive extensively used with personal computers and other data processing devices, the head of each read data signal is recognized on the basis of an address mark. It is a common practice to implement the address mark as a DC-magnetized data erase portion. This conventional scheme, however, has a drawback that when noise is introduced in data erase portions, false read data pulses are generated and obstruct the accurate detection of address marks.
In light of the above, there has been proposed a address mark detection system which counts the duration of a data erase portion and determines, only when it exceeds a predetermined duration, that an address mark has been found. Another system proposed in the past determines that an address mark has been found only when, after the above duration of a data erase portion, a plurality of read data pulses appear continuously for more than a predetermined period of time. On the other hand, Japanese Patent Laid-Open Publication No. 1-122077 discloses a system which causes a counter to output a carry on counting N longer than the duration of a read data pulse and thereby detects an address mark. This system, therefore, does not detect an address mark in the event of noise shorter than the count N. However, this kind of approach is not applicable to a magnetic disk drive of the type rising data erase portions as address marks.